Wednesday, August 22, 2012

Weather Whys Wednesday: Cape Verde Hurricanes

As we head into the heart of hurricane season and with Isaac capturing headlines, let's talk today about Cape Verde storms. When we think of hurricanes, I think most of us envision monster storms rolling across the Atlantic off of Africa all the way to the US, Central America, Caribbean Islands or to a date with fish, Bermuda, the Canadian Maritimes, or like our most recent hurricane Gordon, the Azores. We talked a few weeks back about the Eastern Caribbean and how storms typically behave there. Now let's focus further east.

Google Maps shows you the Cape Verde Islands in
relation to the rest of the Eastern Atlantic Basin

First, why do we call them Cape Verde hurricanes? All because of the geography. As large tropical waves emerge off of Africa, they do so generally between Guinea and Mauritania...or just east of the Cape Verde Islands, which are an island nation west of the African Mainland. As the storms emerge off of Africa, they encounter warm ocean water, which averages over 80 degrees in August. The storms acquire their intensity from a combination of this and the fact that there is no meaningful land mass between the Cape Verde Islands and South America or the Caribbean. So in a perfect world, these systems can continue unabated and strengthen into large, violent hurricanes. As we know, of course, the world is not perfect, and these storms will be steered by troughs, the semi-permanent Azores-Bermuda high over the North Atlantic, among other entities. They will be impacted by anything from enhanced westerlies (like we often see in El Nino events) that add shear to inhibit growth to dust from the Sahara Desert that can dry out the air over them and limit development. But the singular main reason these storms are usually associated with the monsters of the tropics is because of how much time they have to develop.

Typically we end up with approximately two Cape Verde type storms per season. July, August, and September are the prime months for development, but we can see them in June and October as well. Katia and Ophelia were classified as Cape Verde hurricanes last season. Other notable Cape Verde storms in recent years include Ike (2008), Ivan (2004), Frances (2003), Isabel (2003), and Floyd in 1999. Andrew, Hugo, Gilbert, Gloria, Donna, the 1938 Long Island Express, the hurricane of 1933, and the deadliest of all US storms, the 1900 Galveston hurricane were all also Cape Verde storms. So Cape Verde hurricanes are also associated with monster storms because when hitting the US (or elsewhere), many have been monster, deadly, and damaging storms.

Very rough estimates, using NOAA's historical hurricane archive, suggests roughly once every four years or so, one of these will hit the US Mainland. There may be more official or proven statistics out there on that, but I was unable to locate any. Of more interest: Of the 128 storms or remnants of storms to pass within 200 nautical miles of northern Delaware (which includes the entire Delaware Valley, west to Altoona and east to about 150 miles offshore of NJ) since the 1800s, roughly 16 would be classified as "true" Cape Verde storms (forming within about 600 miles of the Cape Verde Islands). About 1/3 of storms or remnants of storms impacting the Delaware Valley or Jersey Shore form east of the Lesser Antilles.

So the bottom line: The longest lasting, occasionally powerful hurricanes we see in the Atlantic are very often the Cape Verde storms. And we'll watch to see if our latest Cape Verde type storm, Isaac, amounts to another notch in the belt.